1. Field of the Invention
The invention relates to an apparatus for detecting a position of an edge of a web by detecting cutoff of ultrasonic-waves transmitting from an ultrasonic-wave transmitter to an ultrasonic-wave receiver, which cutoff is caused by a web being fed between the ultrasonic-wave transmitter and the ultrasonic-wave receiver.
2. Description of the Related Art
In an apparatus for detecting an edge of a web by means of a ultrasonic-wave transmitter and a ultrasonic-wave receiver, ultrasonic-waves are intermittently transmitted from a ultrasonic-wave transmitter such that a period of time in which ultrasonic-waves are not transmitted is longer than a period of time in which reflected ultrasonic-waves are attenuated, in order to prevent a ultrasonic-wave receiver from being influenced by unnecessary reflected ultrasonic-waves.
For instance, in the apparatuses for detecting an edge of a web, suggested in Japanese Patents Nos. 2856066 and 2924641, a peak in signals transmitted from an ultrasonic-wave receiver is detected, and an edge of a web is detected, based on the thus detected peak.
In the apparatus for detecting an edge of a web, suggested in Japanese Patent Publication No. 6-105172 (B1), only third to fifth cycles of signals transmitted from an ultrasonic-wave receiver are selected in order to eliminate influences exerted by ultrasonic-waves transmitted from an ultrasonic-wave transmitter and making a detour to an ultrasonic-wave receiver. A peak is detected in the received ultrasonic-waves in the third to fifth cycles, and an edge of a web is detected, based on the peak.
In an ultrasonic-wave transmitter, even if a driving pulse voltage is stopped to be applied to the ultrasonic-wave transmitter after once applied, there remains reverberant vibration in the ultrasonic-wave transmitter. When an ultrasonic-wave transmitter with small reverberation vibration is used, an amplitude of an output signal transmitted from a ultrasonic-wave receiver, influenced by reverberant vibration and unnecessary reflected ultrasonic-waves, is smaller than an amplitude of direct waves. Hence, by using a ultrasonic-wave transmitter with small reverberant vibration, it would be possible to detect an edge of a web, based on a peak of output signals transmitted from a ultrasonic-wave receiver.
In contrast, when a ultrasonic-wave transmitter with high reverberant vibration is used, an amplitude of an output signal transmitted from a ultrasonic-wave receiver on receipt of ultrasonic-waves transmitted from a ultrasonic-wave transmitter while a driving pulse voltage is being applied to the ultrasonic-wave transmitter is sometimes greater than an amplitude of an output signal transmitted from a ultrasonic-wave receiver on receipt of ultrasonic-waves caused by reverberant vibration after application of a driving pulse voltage to the ultrasonic-wave transmitter has been stopped.
However, the ultrasonic-waves caused by reverberant vibration are unstable, and hence, is likely to be influenced by reflected ultrasonic-waves. Thus, it is necessary to sample only ultrasonic-waves transmitted from a ultrasonic-wave transmitter while a driving pulse voltage is applying to the ultrasonic-wave transmitter.
FIG. 1 illustrates waveforms of signals transmitted from a ultrasonic-wave receiver. Specifically, FIG. 1-(A) illustrates a waveform of a gate signal, FIG. 1-(B) illustrates a waveform Vt0 of an output signal transmitted from a ultrasonic-wave receiver at a temperature Ta equal to a certain temperature T, FIG. 1-(C) illustrates a waveform Vt1 of an output signal transmitted from a ultrasonic-wave receiver at a temperature Ta higher than the temperature T, and FIG. 1-(D) illustrates a waveform Vt2 of an output signal transmitted from a ultrasonic-wave receiver at a temperature Ta lower than the temperature T.
As illustrated in FIG. 1-(C), the higher the temperature Ta is, earlier the output signal starts being transferred, and as illustrated in FIG. 1-(D), the lower the temperature Ta is, later the output signal starts being transferred. That is, a velocity at which ultrasonic-waves are transferred in air is dependent on ambient temperature.
Hence, in the apparatus suggested in the above-mentioned Japanese Patent Publication No. 6-105172 (B1) in which a peak is detected when a certain period of time has passed after ultrasonic-waves had been transmitted, if ambient temperature much varies, a velocity at which ultrasonic-waves are transferred in air also varies, and hence, an output signal transmitted from a ultrasonic-wave receiver is shifted relative to a specific duration, that is, a period of time in which a gate signal is applied. As a result, as a peak is detected a waveform Vt1 or Vt2 both different from a waveform Vt0 which should be detected.
To avoid this problem, U.S. Pat. No. 5,126,946 discloses the apparatus designed to include a thermister for selecting a sampling area in accordance with ambient temperature.
However, the apparatus is useful only when ambient temperature gradually varies. The apparatus is not useful for air convection in which temperature rapidly varies, and further, not useful when a temperature profile is not uniform.
In view of the above-mentioned problems in the conventional apparatuses, it is an object of the present invention to provide an apparatus for ultrasonically detecting an edge of a web being fed between a ultrasonic-wave transmitter and a ultrasonic-wave receiver which apparatus is capable of transmitting and receiving ultrasonic-waves for accurately detecting an edge of a web without being influenced by reverberation vibration and reflected ultrasonic-waves, even if a time necessary for ultrasonic-waves to reach a ultrasonic-wave receiver from a ultrasonic-wave transmitter varies.
It is also an object of the present invention to provide a method of ultrasonically detecting an edge of a web being fed between a ultrasonic-wave transmitter and a ultrasonic-wave receiver which method of capable of doing the same.
In one aspect of the present invention, there is provided an apparatus for ultrasonically detecting an edge of a web, including a ultrasonic-wave transmitter transmitting ultrasonic-wave pulse train, a ultrasonic-wave receiver arranged in facing relation to the ultrasonic-wave transmitter, receiving the ultrasonic-wave pulse train, and converting the received ultrasonic-wave pulse train into electric signals, the web being fed between the ultrasonic-wave transmitter and the ultrasonic-wave receiver, a rectifying circuit for rectifying the electric signals, a low-pass filter circuit for smoothing output signals transmitted from the rectifying circuit, a first sample-holding circuit for sampling an output signal transmitted from the low-pass filter circuit, at first timing, a second sample-holding circuit for sampling an output signal transmitted from the low-pass filter circuit, at second timing later than the first timing, a third sample-holding circuit for sampling an output signal transmitted from the first sample-holding circuit, at the second timing, and a differentially amplifying circuit for calculating a difference between an output signal transmitted from the second sample-holding circuit and an output signal transmitted from the third sample-holding circuit.
On receipt of ultrasonic-wave pulse train from the ultrasonic-wave transmitter, the ultrasonic-wave receiver converts the received ultrasonic-waves into electric signals, which are rectified by the rectifying circuit. The rectified electric signals are smoothed by the low-pass filter circuit. The first sample-holding circuit samples an output signal transmitted from the low-pass filter circuit at first timing. The second sample-holding circuit samples an output signal transmitted from the low-pass filter circuit, at second timing later than the first timing, and the third sample-holding circuit samples an output signal transmitted from the first sample-holding circuit, at the second timing. The differentially amplifying circuit calculates a difference between an output signal transmitted from the second sample-holding circuit and an output signal transmitted from the third sample-holding circuit.
An amplitude of an output signal transmitted from the ultrasonic-wave receiver almost proportionally increases while the ultrasonic-wave receiver receives ultrasonic-waves directly driven by driving pulses applied to the ultrasonic-wave transmitter. The inventors experimentally found out that signals obtained by rectifying output signals transmitted from the ultrasonic-wave receiver and further smoothing the signals by mean of the low-pass filter circuit linearly increase. Hence, by taking two samples in a range in which the smoothed signals linearly increase, and calculating a level difference between the two samples, it would be possible to accurately detect an edge of a web without being influenced by reverberant vibration and reflected ultrasonic-waves, even if a time necessary for ultrasonic-waves to reach the ultrasonic-wave receiver from the ultrasonic-wave transmitter varies due to temperature variance.
The two sampling points are selected in a range in which the smoothed electric signals linearly increase, that is, a range in which the ultrasonic-wave receiver receives ultrasonic-waves directly driven by driving pulses, even if a time necessary for ultrasonic-waves to reach the ultrasonic-wave receiver from the ultrasonic-wave transmitter varies due to temperature variance in an expected range.
In another aspect of the present invention, there is provided a method of ultrasonically detecting an edge of a web, by detecting cutoff of ultrasonic-waves transmitted from a ultrasonic-wave transmitter to a ultrasonic-wave receiver, the cutoff being caused by a web being fed between the ultrasonic-wave transmitter and the ultrasonic-wave receiver, including converting ultrasonic-waves received by the ultrasonic-wave receiver, into electric signals, and rectifying the electric signals, smoothing the thus rectified electric signals, sampling the smoothed electric signals at first timing by means of a first sample-holding circuit, sampling the smoothed electric signals at second timing later than the first timing by means of a second sample-holding circuit, sampling an output signal transmitted from the first sample-holding circuit, at the second timing by means of a third sample-holding circuit, and calculating a difference between an output signal transmitted from the second sample-holding circuit and an output signal transmitted from the third sample-holding circuit.
The method may further include selecting the first and second timings out of a period of time in which the smoothed electric signals linearly increase.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.